Multiple beam charged particle optical system

Abstract

The invention relates to a multiple beam charged particle optical system, comprising an electrostatic lens structure with at least one electrode, provided with apertures, wherein the effective size of a lens field effected by said electrode at a said aperture is made ultimately small. The system may comprise a diverging charged particle beam part, in which the lens structure is included. The physical dimension of the lens is made ultimately small, in particular smaller than one mm, more in particular less than a few tens of microns. In further elaboration, a lens is combined with a current limiting aperture, aligned such relative to a lens of said structure, that a virtual aperture effected by said current limiting aperture in said lens is situated in an optimum position with respect to minimizing aberrations total.

The invention claimed is: 
     
       1. Multiple beam charged particle optical system, comprising:
 a beam source for emitting a diverging charged particle beam; 
 an aperture plate comprising a plurality of apertures and arranged for splitting said charged particle beam into a plurality of non-parallel charged particle beamlets, each beamlet having an optical axis; and 
 a planar electrostatic lens structure comprising a corresponding plurality of lens holes for said plurality charged particle beamlets; 
 said aperture plate and said electrostatic lens structure being aligned and arranged in a path of said plurality of non-parallel charged particle beamlets, 
 wherein corresponding apertures of the aperture plate and the electrostatic lens structure are aligned with said optical axes of the corresponding beamlets, such that for each beamlet a virtual aperture of the electrostatic lens structure is symmetrical along an optical axis of a corresponding lens in said planar electrostatic lens structure. 
 
     
     
       2. System according to  claim 1 , wherein the electrostatic lens structure comprises a first electrode and a second electrode, said first electrode and said second electrode being provided with a plurality of apertures corresponding to the plurality of apertures of the aperture plate. 
     
     
       3. System according to  claim 2 , wherein the first electrode and/or second electrode is substantially planar. 
     
     
       4. System according to  claim 2  or  claim 3 , wherein the corresponding apertures of the first electrode, the second electrode and/or of the aperture plate are aligned such that their corresponding apertures are shifted with respect to each other relative to a central optical axis of the electrostatic lens structure. 
     
     
       5. System according to any one of  claims 1 - 3 , wherein the plurality of charged particle beamlets comprises charged particle beamlets at an angle to a central optical axis of the electrostatic lens structure. 
     
     
       6. System according to any one of  claims 1 - 3 , wherein the electrostatic lens structure further comprises one or more further electrodes provided with apertures aligned with said optical axes of said beamlets. 
     
     
       7. System according to any one of  claims 1 - 3 , wherein a diameter of an aperture of the electrodes of the electrostatic lens structure and/or of the aperture plate is a function of a distance of the relevant aperture to a centre point of said electrodes or aperture plate. 
     
     
       8. System according to any one of  claims 1 - 3 , wherein a distance between adjacent apertures of the electrodes of the electrostatic lens structure and/or of the aperture plate, is a function of the distance of the relevant aperture to a centre point of said electrode or aperture plate. 
     
     
       9. System according to any one of  claims 1 - 3 , wherein said aperture plate and said planar electrostatic lens structure are integrated in one electrode; or form a two electrode microlens array made of a SOI wafer, or a two electrode microlens array by bonding of two wafers. 
     
     
       10. System according to any one of  claims 2 - 3  wherein said plurality of apertures of the aperture plate and the first or electrode of said planar electrostatic lens structure are made of one piece of wafer.